Rochelle salt piezoelectric crystal apparatus



A 11, 1942- w P. MASON 2,292,385

RDCHELLE SALT PIEZOELECTRIC CRYSTAL APPARATUS Filed May 9, 1941 INVENTORV W F. MASON ATTORNEY Patented Aug. 11, 1942 UNITED. STATES PATENTOFFICE ROCHELLE SALT PIEZOELECTRIC CRYSTAL APPARATUS Warren P. Mason,West Orange, N. J., assignor to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application May 9, 1941,Serial No. 392,688

20 Claims. 01. 171-327) This invention relates to piezoelectric crystalapparatus and particularly to piezoelectric Roelement having one or moreuseful low frequency modes of motion that may be utilized either aloneor simultaneously, or coupling with other modes of motion therein.

Another object of this invention is to provide a Rochelle salt crystalelement having a plurality of simultaneously useful and indewithoutinterference pendently controlled frequencies that may be substantiallyuncoupled with each other and free from spurious or undesiredfrequencies.

Another object of this invention is to provide Rochelle salt crystalelements of such an orientation that the longitudinal length and widthmodes of motion thereof may be uncoupled to the face shear mode ofmotion thereof.

Another object of this invention is to reduce the number and the cost ofcrystals used in electric wave filter systems and other wavetransmission networks, and to take advantage of the high piezoelectricactivity and low cost of Rochelle salt.

Rochelle salt piezoelectric crystal elements generally maybe excited inmany different modes of motion such as, for example, extensional orlongitudinal modes, flexural modes, and shear modes of motion. Whencrystal elements are to be applied to filter systems, for example, it

is generally desirable to have. all of the undesired or extraneous modesof motion therein uncoupled with and considerably higher, or lower, infrequency than the. desired main mode or modes of motion of the crystalelement since otherwise the extraneous resonance frequencies therein mayintroduce undesirable frequencies or pass bands in the filtercharacteristic. According- 1y, it is often desirable in filter systemsand elsewhere that the desired main mode or modesiof motion of .acrystal element be substantially uncoupled to other modes ofmotion andindependently controlled in order that such mode or modes of motion maybe given any desired frequencYvalues to obtain prescribed frequencycharacteristics.

In accordance with this invention, wave filters and other systems maycomprise as a component element thereof, a single piezoelectric crystalelement of Rochelle salt which may be adapted to vibrate simultaneouslyin a plurality of substantially uncoupled modes of motion in order toprovide either separately or simultaneously a plurality of usefuleffective resonances which may be independently controlled and placed atpredetermined frequencies of the same or different values for use in anelectric wave filter or elsewhere.

The crystal element may be a Rochelle salt I crystal plate of suitableorientation with respect to the X, Y and Z axes thereof, and of suitabledimensional proportions, and provided with a suitable electrodearrangement and connections for separately driving either, orsimultaneously driving both, of two uncoupled modes of motion thereinand independently controlling the relative strengths of such resonances.

In particular embodiments, the orientation of the crystal element may bethat of an X-cut or a Y-cut or a Z-cut Rochelle salt crystal platerotated in effect about its X-axis or Y-axis or Z-axis thicknessdimension respectively. The

width dimension of itsmajor surfaces and the length dimension thereofmay be of selected values in order to obtain therefrom, separately orsimultaneously, either or both of two useful independently controlledresonant frequencies resulting from two independently controlled facemodes of motion, one particular set of which is described herein as thefundamental width axis dimension longitudinal or extensional mode andthe other as the second harmonic length axis longitudinal or extensionalmode. Both the length and width longitudinal modes of motion referred toare in the major plane of the crystal element, and due to the crystalorientation selected may have no mechanical coupling with each other orwith the face shear mode vibration.

Such Rochelle salt crystal elements when provided with suitableelectrodes may be connected into a filter circuit in such a way that oneof the resonances of each crystal element is effective in the linebranch and another .of the resonances is effective in the diagonalbranch of the lattice portion of the equivalent network thereof, inorder to obtain filter circuits using a single crystal which areelectrically equivalent to circuits requiring two crystals, therebyreduc ing the number and cost of crystals therein. Such Rochelle saltcrystal elements may be utilized, for example, in either balanced orunbalanced filter structures such as those disclosed, for example, in W.P. Mason U. S. Patent 2,271,870, granted February 3, 1942, on myapplication Serial No. 303,757, filed November 10,

of frequency, there is less absolute shift in the pass band duetotemperaturechange and there fore temperature control often is notneeded at the lower values of frequency and electric wave filters may bemade using Rochelle salt crystal elements as the vibrating elementsthereof, with characteristics nearlyas good as those obtained when usingquartz crystal elements. However, due tothe relatively high temperaturefrequency coefiicient of Rochelle salt, it is often desirable to havetemperature control to about 1 or 2 C. to hold the pass bands to theirrequired frequency. By the use of both wet and dry Rochelle saltmaterial placed in the enclosing container, the Rochelle salt vibratorycrystal may be preserved indefinitely, or for a long period of timewithout change in the characteristics thereof.

For a clearer understanding of the nature of this invention and theadditional advantages, features and objects thereof, reference is madeto the following description taken in connection with the accompanyingdrawing, in which like reference characters represent like or similarParts and in which:

Figs. 1, 2 and 3 are respectively perspective views of X-cut, Y-cut andZ-Cut type pie oe ctric Rochelle salt type crystal .elements inacoordance with this invention, and illustrate particularly theorientation thereof with respect to the X, Y and Z axes of the Rochellesalt crystal material from which the crystal elements may be cut;

Figs. 4 to 8 are views illustrating types of electrodes and connectionstherewith which may be utilized with any of the Rochelle salt crystalelements of Fig.1, 2.01 8 to drive the crystal element separately ineither or simultaneously in both of two independently controlledlongitudinal modes of motion, fundamental or harmonic, in order toobtain the desired resonance frequency or frequencies.

Fig. 4 is a perspective view of an electrode arfundamental longitudinalmode of motion along,

. either the length or width dimension thereof;

. Figs; 5 and 6 are perspective views of electrode arrangements, thatmay be used to drive the crystal element of 1, 2 or 3 in the secondharmonic longitudinal length mode of motion and the fundamentallongitudinal width mode of ofFig.5;and 1 i i Fig. 8 is a schematicdiagram illustratingan example of unbalanced filter connections that maybe used in connection with the electrodes of the crystal element-of Fig.6.

rangement that may be used to drive the piezo- 4 electric crystalelement of Fig. 1, 2 or 3 in the his specification follows theconventional terv minology' as applied to crystalline Rochelle salt, I

which employs three orthogonal or mutually perpendicular a, b and c axesor X, Y and Z axes,

M respectively, as shown in the drawing. to d signate an electric axis,a mechanical axis andan optic axis, respectively, of piezoelectricRochelle salt or sodium potassium tartrate crystal material, and.whichemploys three orthogonal axes X'-, Y and Z to designate the directionsof axes of a piezoelectric body angularly oriented with respect to suchX, Y and Z axes thereof. Where the orientation is obtained in efiect bya single rotation of the Rochelle salt crystal element, the rotationbeing in effect substantially about the thickness dimension axis X, Y orZ of the piezoelectric body as illustrated in Figs. 1, 2 and 3,respectively, the orientation angles, respectively, a=substantially 4956', 6=42 .26, and =an angle of substantially 45 degrees between the Xand Y axes, designate in degrees the effective angular position of thelength axis dimension L of the crystal plate as measured from one of theother two X, Y and Z axes. The relation of the X, Y and Z axes to theouter faces of a grown Rochelle salt crystal body are illustrated in W..P. Mason U. S. Patent 2,178,146, dated 'October 31, 1939. Rochelle saltbelongs to the rhombic hemihedral class of crystals and has threeorthogonal or mutually perpendicular axes generally designated as the a,b and c axes or the X, Y and Z axes, respectively.

Referring to the drawing, Figs. 1, 2 and 3 rep-- resent perspectiveviews of thin bare piezoelectrio Rochelle salt type crystal elements I,2 and 3 cut from crystalline Rochelle salt free from defects and madeinto a plate of substantially rectangular parallelepiped shape with itsmajor surfaces having a length or longest dimension L and a widthdimension W- which is perpendicular to the length dimension L, thethickness or thin dimension T between the major surfaces beingperpendicular to the other two dimensions L and W. In accordance withthe particular mode or modes of motion selected, the final widthdimension W of the Rochelle salt crystal element 8, 2 or 3 of Figs. 1, 2and 3 may be made of suitable value according to the desired resonantfrequency. The width dimension W also may be related to the lengthdimension L in accordance with the value of the desired resonantfrequency. The thickness dimension T may be of the order of 1 millimeteror any other suitable value, for example, to suit the impedance of thecircuit in which the crystal element l, 2 or 3 of Figs. 1, 2 and 3 maybe utilized.

As shown in Fig. 1, the length dimension L of the X-cut type crystalelement l illustrated in Fig. 1 lies along a Y axis in the plane of themechanical axis Y and the optic axis Z of the Rochelle salt crystalmaterial from which the element I is cut and is inclined at an angle of0: degrees with respect to said Y axis, the angle on being one of thevalues in the region of substantially 49 degrees 56 minutes (49 56').Inc major surfaces and the major plane of the R0- chelle salt crystalelement I of Fig. 1 are disposed parallel or nearly parallel with respwtto the plane of the Y and Z axes, the lengthdi mension L and the widthdimension W lying 1 along the Y"aXis and the Z axis, respectively,

axis and the optic axis Z respectively. The axis Y is accordingly theresult of a single rotation of the length dimension L about the X axis}oz degrees. It will be noted that the crystal ele'- ment I of'Fig. 1 isin effect an X-cutRochelle' salt crystal plate rotated degrees about thex axis.

salt piezoelectric crystal element 2 having its longest or lengthdimension L along the X' axis and inclined at an angle of0=substantially 42 26' with respect to the X axis, the major surfaces ofthe crystal element 2 being parallel or nearly parallel to the plane ofthe Z axis and the X axis.

Fig. -3 represents a Z-cut type Rochelle salt crystal element 3 havingits length or longest dimension L along the X axis and inclined at anangle 0 which may be any angle intermediate the X and Y axes, the majorsurfaces of the crystal element 3 being parallel or nearly parallel tothe plane of the X and Y axes.

The orientations illustrated in Figs. 1, 2 and 3 accordingly representX-cut, Y-cut and Z-cut type Rochelle salt piezoelectric crystal elementsI, 2 and 3, respectively, which may be adapted for independentlycontrolled longitudinal length L and longitudinal width W modevibrations, and also other low frequency or face mode Vibrations, whichmay be utilized either alone or simultaneously, according to thearrangement of the electrodes and connections that are used therewith,and the dimension-frequency constants that are selected therefor.

Suitable conductive electrodes, such as the crystal electrodes of Fig.4, 5 or 6, for example, may be placed on or adjacent to or formedintegral with the opposite major surfaces of the crystal plate I, 2 or 3of Figs. 1, 2 and 3 in order to apply electric field excitation to theRochelle salt plate I, 2 or 3 which may be vibrated alone orsimultaneously in a desired width W fundamental longitudinal mode ofmotion and/or the length L longitudinal fundamental or harmonic mode ofmotion at independentlycontrolled resonant response frequencies whichdepend upon different sets of dimensions involving the width dimension Wand the length dimension L, the fundamental longitudinal length andwidth mode frequencies being roughly from 117 to 186 kilocycles persecond per centimeter of the width carbon, silver, gold, platinum,aluminum or other suitable metal or metals deposited upon the surfacesby painting, spraying, or evaporation in vacuum for example, or by othersuitable proc- When the Rochelle salt crystal plate I or 2 has anorientation angle of a=si1bstantially 49 56 with respect to the Y axisas illustrated in Fig. 1, or 0=42 26 with respect'to the X axis asillustrated in Fig. 2, the length L longitudinal fundamental'mode ofmotion or harmonic thereof is mechanically uncoupled to other modes suchas the fundamental longitudinal or extensional mode of motion along thewidth dimension W of the crystal plate I or 2 and the lattermay be usedsimultaneously, without coupling to, for example, the second harmonicextensional mode along length dimensional L.

Fig. 2 isa perspective view of a Y-cut Rochelle tion simultaneously, onealong the length dimension L and the other along the width dimension W,it is necessary that the crystal element have a piezoelectric constantwhich will generate a longitudinal motion along the width dimension Wand also anotherpiezoelectric constant that will generate a longitudinalmotion along the length dimension L.

In the case of the X-cut type Rochelle salt crystal element Iillustrated in Fig. 1, the requirement of suitable piezoelectricconstants (1'12 and (1'13 may be met when the length dimension L isinclined at any suitable angle a between the Y and Z axes in the YZplane, the YZ plane being parallel or nearly parallel to the major planeand the major surfaces of the Rochelle salt crystal element I of Fig. 1.

As indicated by Equations 31 and 32 of my paper A dynamic measurement ofthe elastic. electric and piezoelectric constants of Rochelle saltpublished April 15, 1939, in Physical Re- View, volume 55, page 775, thepiezoelectric constants diz and d'is involved in the longitudinal orextensional mode vibrations along the length dimension L and along thewidth dimension W,

respectively, of the X-cut Rochelle salt crystal element I illustratedin Fig. 1 are equal to:

The constants d'iz and dia are of equal value as shown by Equation 1 andreach their maximum values when the angle a 45 or when the lengthdimension L of the X-cut type Rochelle salt crystal element I of Fig. 1is inclined 45 with respect to the Y and Z axes thereof, the major sufaces thereof being parallel to the plane of such Y and Z axes.

When it is desired to; independently use these tWo desired modes, theangle a. may be made 49 56 to provide for a zero or no coupling betweeneither the length or the width longitudinal modes and the face shearmode, and thereby to avoid a resulting mechanical coupling between thedesired second, or other harmonic, of the length longitudinal mode alongthe length axis Land the desired fundamental of the width longitudinalmode along the width axis dimension W. Since any coupling directlybetween the second harmonic longitudinal mode along the length dimensionL and the fundamental longitudinal mode. along the width dimension Wcancels itself out, it is not necessary to provide for a zero couplingdirectly between these two modes.

When the length dimension L of the X-cut type Rochelle salt crystalelement I of Fig. 1 is inclined at an angle of a=substantially 49 56with respect to the Y axis, the coupling coeflicient S'24 representingthe coupling between the face shear mode and both the length L and widthW longitudinal modes becomes zero.

As indicated by Equation 33 of my Physical Review paper hereinbeforereferred to, the equation for the value of s'24 is:

S 4=SiII 2a where I S23=1.03 x 10 822:3.495 x 10- To obtain the twolongitudinal modes of mothe Y axis, as illustrated in Fig. 1.

Substituting these values in the above Equation 2, the angle a. forwhich 8'24 becomes zero and vanishes is u=49'56', a being the anglebetween the' length dimension L and the nearest Y axis, as illustratedin Fig. 1.-

While the maximum values for the piezoelectric constants dig and d'iaoccur when the angle (1:45, the angle of -a='49 55' is near enoughthereto to obtain good values of piezoelectric constants (1'12 and (1'13and, moreover, at the same time obtain the desired longitudinal modesalong the length dimension L and the width dimension W withoutinterference or coupling with each other. Such an X-cut type Rochellesalt crystal element i of Fig. 1 also has-a'strong electromechanicalcoupling'which varies somewhat with motion referred to cannot be usedsince the piezoelectric constants d'aa and d'ai controlling these modesare both of zero value at the zero coupling angles of =0 degrees or =90degrees, the angle being measured between the length or longestdimension L of the crystal element 3 and the nearest X axis thereof, asillustrated in Fi '3.

For the Z-cut type Rochelle salt crystal element 3 of Fig. 3, thepiezoelectric constants (1'32 and dai are:

d32= sin 2; 'd sin 2 (5) and the coupling constant S'za for the lengthor width longitudinal mode of motion and the face shear mode of motionis:

where o is the angle between the length axis dimension L and the Xcrystallographic axis, as

illustrated in Fig. 3. su=5.1s 1o with temperature, and with the 0 angleequal to substantially 42 26', the length L longitudinal mode of motionand width W longitudinal mode of motion are substantially free fromcoupling with each other andwith any other modes of motion therein.

For the Y-cut type Rochelle. salt crystal element 2 illustrated in Fig.2, the piezoelectric constants controlling the longitudinal mode vibrations along the, length dimension L and along the width dimension W are:

sin 20 1 (3) The coupling coeiiicient or constant S'it fo thelongitudinal length mode is:

' =sin 20 where 0==the angle between X axis and the X crystallographicaxis.

Sic -2.11 X 10" (1 Sill (1 23 Substituting these values in Equation 4,the angle 0 for which 8'15 becomes zero and vanishes is 0=42 26 asillustrated in Fig. 2.

-When the angle 6:42 26', as illustrated in Fig. 2, there issubstantially no coupling between either the longitudinal mode along thelengthaxis dimension L or the longitudinal mode along mode width -W andlength L vibrations of thekind useful for a doubly resonantcrystaielement. However, in such a Z-cut type crystal element 3, the zerocoupling angles of =0 or 90 the length axis L along the degrees involvedin the two modes of longitudinal S22=3.495 X 10- Sec: 10.08 X 10-Inserting these values in Equation 6, the S'ze coupling constantvanishes only at =0 degrees and =90 degrees. Since the piezoelectricconstants (1'32 and d'si are both zero at .both of these angles, a Z-cuttype Rochelle salt crystal element 3 having its length or longestdimension L disposed or inclined at an angle of '=0 or de-'- grees withrespect to its major plane X axis is not useful at these particularangles as a doubly resonant crystal element involving the secondharmonic of the length L longitudinal mode of motion and the fundamentalof the width W longitudinal mode of motion. However, such a Z- cut typeRochelle salt crystal element 3 may be utilized at angles other thanthose of 0 or 90 degrees and particularly at the angle of 5:45 degreesfor a single longitudinal mode of motion.

. a The principal modes of interest that are par-' ticularly consideredherein in connection with the- X-cut,. Y-cut and Z-cut type crystalorientations illustrated in Figs. 1, 2 and 3 are the width longitudinalmode of motion'along the width W axis and also the length longitudinalmode of motion, fundamental and harmonic, along the length dimension Lof the crystal elements I, 2 and 3 illustrated in Figs. 1, 2 and 3. Thelongitudinal width W mode vibration, fundamental or harmonic, operatesto alternately extend and shorton the width dimension W of the crystalelement l. 2 or 3 about a nodal line which for the fundamental vibrationextends along the center line length dimension Lof the crystal elementI, 2 or 3. Similarly, the longitudinal length L mode of motion,fundamental or harmonic, operates to alternately extend and shorten thelength serted in very small inden nodal line involved in the widthlongitudinal 'mode' of motion, the crystal elements of Fig. 1, 2 or 3may be mounted there or near the nodal point or points without dampingor interfering with the simultaneous operation of either the width Wlongitudinal mode vibration or the length L longitudinal mode vibration.In the case of the second harmonic of the length L longitudinal modevibration, the two nodal point regions on each of the major surfaceswould be located on the center line length dimension L of the crystalelement at points spaced about 0.25 of the length dimension L from eachend thereof. Accordingly, at such nodal points the crystal element ofFig. 1, 2'or 3 may be mounted by rigidly clamping it there between twopairs of oppositely disposed clamping projections of small contact areawhich may be there placed or intions or depressions cut or provided atthe foul nodal points of the crystal element. Such small depressions maybe cut in the major surfaces of the crystal element at the nodal pointsthereof and may have adepth of about 0.05 millimeter and a diameter ofabout 0.4 millimeter as measured on the major surfaces of the crystalelement.

The relative values of the resonance frequencies associated with thewidth W longitudinal mode vibration and with the length L longitu- =45respectively, as illustrated in Figs. 1, 2 and 3, the fundamental of thewidth W longitudinal mode frequency has a frequency-dimension constantrespectively of 160, 118 or 186 kilocycles per second per centimeter ofwidth dimension W independent of the dimensional ratio of the width Wwith respect to the length L. The fundamental of the length Llongitudinal mode vibration of such X-cut, Y-cut, and Z-cut crystalelements I,.2 and 3 having the particular orientation angles mentionedand illustrated in Figs. 1, 2 and 3,

dimension W with respect to the length dimension L'is in the region ofabout 0.5; and in this region of special interest, the resonances ofthese two modes are substantially uncoupled or only very loosely coupledwhen the 0: angle of the Rochelle salt crystal element I has a value ofsubstantially 49 56 as illustrated in Fig. 1 or the 0 angle of thecrystal element 2 of Fig. 2' has.

a value of substantially 42 26'.

Accordingly, when the dimensional ratio of W/L is in the region of 0.5and the orientation is that of Fig. 1 or.,2, the frequencies of thesetwo independent modes of vibration may be placed close together butremain sufliciently uncoupled to provide simultaneously twoindependently controlled frequencies from the same Rochelle saltcryst'al element, which may be usefully employed in a filter system, forexample, to give conveniently frequencies of the order of to 200kilocycles per second, for example, within a range of frequencies from50 or less to 500 or more kilocycles per second.

The fundamental frequency in kilocycles per second of the longitudinalwidth W mode vibration is given by the relation:

respectively, for the crystal elements I, 2 and 3 where W is the lengthvalue of the width dimension W of the crystal element I, 2 or 3 of Fig.1, 2 or 3 expressed in centimeters.

The fundamental frequency in 'kilocycles per second of the longitudinallength L mode vibration is given by the same relation:

160 117.5 186 f T T" T where L is the length dimension L of the crystalplate expressed in centimeters. The foregoing frequency-dimensionconstants respectively represent values that obtain when the X-cut,Y-cut and Z-cut crystal elements I, 2 and 3 have the orientation anglesof a=49 56', 6=42 56 and =45, as illustrated in Figs. 1, 2 and 3.

Figs. 4, '5 and 6 illustrate forms of electrode arrangements which maybe utilized to drive any of the crystal elements of Fig. 1,. 2 or 3. Asillustrated in Fig. 4; a single pair of electrodes 9 and I5 may be usedto drive the crystal element I, 2 or 3 in either the width Wlongitudinal fundamental mode vibration or the fundamental of thelongitudinal length L mode of motion to obtain separately, but notsimultaneously, either of two independently controlled longitudinalfundamental mode resonance frequencies of a desired value.

More particularly, Fig. 4 is a perspective view of the crystal elementI, 2 or 3 of Figs. 1, 2 and 3 provided with a pair of oppositeelectrodes 9 and I5 which may be utilized to usefully operateseparately, but not together, in the crystal element I, 2 or 3 of Figs.1 to 3, the fundamental of the longitudinal length L mode of motion orthat of the longitudinal width W mode of motion at a frequency of 117 to186 kilocycles per second per centimeter of length dimension L or widthdimension W. For this purpose the electrodes 9 and 15 may partially orwholly cover the major surfaces of the crystal element L2 or 3 and maybe supported and connected in circuit by means of suitable conductivemembers disposed in contact with each of the electrodes 9 and I5. It

- will be understood that Rochelle salt crystal eleother modes ofmotion; The harmonic frequency may be of any desired order and may beobtained by the use of electrode arrangements known in the art inconnection with quartz longitudinal mode crystal elements, such as, forexample, It may be approximately 1 millimeter in width those shown inMason U. S. Patent 2,185,599 dated January 2, 1940.

As shown in Fig. 5, the second harmonic of the longitudinal length Lmode of motion may be driven by means of two .pairs of electrodes it, M,2 and it placed on both of the major surfaces of the crystal element ofFig. 1, 2 or 3; and also with suitable connections, the fundamental ofthe the result that the two useful and independently controlledresonance frequencies of the crystal element may be made toappear'simultaneously.

-More particularly, as illustrated in Fig. 5, the

Rochelle salt crystal element 6, 2 or 8 of Fig. 1 2 or. 3 may beprovided with four equal-area electrodes in, H, I2 and i3, two'of theelectrodes it and it being placed on one major surface of the crystalelement with a centrally located narrow transverse split, gap ordividing line i 'therebetween, and the other two electrodes l2 and l3,being oppositely disposed and placed on the opposite major surface ofthe crystal element and separated with a similar narrow and oppositelydisposed split or dividing line 7 therebetween, the dividing lines "iextending generally in the direction of the width W axis of the crystalelement according to the value of the angle selected between thedirection of the dividing line 7 and the direction of the lengthdimensionL. The gap or separation between the electrode coatings orplatings on each of the major surfaces-of the crystal element may be ofthe order of about 0.3 millimeter, with the center line of such splits 7in the platings It to 53 on the opposite sides of the crystal platebeing aligned with respect to' each other.

'Fig. 7 is a schematic diagram illustrating an example of balancedfilter connections which may be used with the electroded crystalarrangement of Fig. 5 in order to obtain a filter system comprising asingle Rochelle salt crystal element having two independently controlledand simultaneously effective resonances which may be placed at desiredfrequencies, one of which may appear in the line branch of theequivalent lattice and the other in the diagonal branch thereof, asdescribed more fully in connection with Figs. 2 and 3 of the Masonapplication Serial No. 303,757 referred to hereinbefore.

The balanced circuit of Fig. 5 may be converted into an unbalancedfilter structure by intercon necting the two electrodes l2 and I3 on oneof the major surfaces of the crystal element. In

schematically in Fig. 8 and as described more fully in connection withFigs. 6 and 7 of the Mason application Serial No. 303,757 hereinbeforereferred to.

As illustrated in Fig. 8, to reduce the magnitude of the shuntingcapacitance appearing in the line branch of the lattice portion, anarrow grounding strip M of metallic or conductive coating or platingmay be placed on one major surface of the crystal element between theelectrodes Ill and II. The strip M may extend around one and may beplacedbetween and separated from the two electrodes it and II on thesame major surface of the crystal element l in order to provideshielding and to reduce stray capacities to a The strip of plating M mayex-- tend from one major surface continuously over and around one edgeonly or both edges of the crystal plate 8 .to the opposite major facethereof a where it may make contact with the integral electrode l5 onthat surface. It will be noted that in order to drive the electrodedcrystal element of Fig. 5 or 6 in the second'harmonic of thelongitudinal length mode of motion, one half of the'crystal plate ismade of opposite polarity to that of the other half, as indicated by theand signs in Figs. 5 and 6, and that this may be accomplished byutilizing a crystal element having divided metallic coatings it and iiplaced on one of its major surfaces and connected in the form of a Tnetwork, for example, as illustrated in'Fig. 8. Inductance coils may beadded in the usual manner in series or in parallel with the network ofFig. 8 to produce broad band low or high impedance filters, for example.In order that the crystal impedance may appear in both arms of thelattice structure of Fig. 8-, one mode is driven when the terminals 2!and 23 are both of same polarity, and the other mode is driven whenthese terminals 2! and 23 are of opposite polarity. Since both modes aresubstantially uncoupled they may produce simultaneously two independently controlled resonances of predetermined frequencies ofdesiredvalues;

In order to control the relative impedance levels of the two desiredcrystal resonances, the crystal electrodes associated with one half ofthe major surface or surfaces of crystal element i, 2 or 3 of Figs. 5and 6 may be extended to cover a portion of the other half thereof. Thismay be done, for example, by adjustment of the angular position of theelectrode dividing line i with respect to the length dimension L. The

angle may be any desired value over a wide range of angles. Thisadjustment does not materially affect the impedance of the longitudinalwidth W mode resonance, but with decreasing values for the 90-degreeangle shown in Figs. 5 and 6 will increase the impedance level of andout down the drive on, second harmonic length L mode resonance, withoutmaterially afiecting the. impedance of the width W longitudinal moderesonance. Thus, by changing the angle of inclination of the split ordivision line 7 between the electrodes It] and H with respect to thelength dimension L of the crystal element illustrated by a 90-degreeangle in Figs. 5 and 6, the internal capacity associated with the'second harmonic of the longitudinal length L mode of motion, which isnearly a maximum value when the angle equals 90 degrees as shown inFigs. 5

' and 6, may be varied and adjusted to a desired edge of thecrystal'element I to the opposite major surface thereof where it may beelectrically connected to the large electrode I5. The ground strip ingis used, opposite conductive clamping projections may resilientlycontact the electroded crystal element at its nodal points only in orderto support and to establish individual electricalconnections therewith.

Alternatively, instead of being mounted by clamping, the electrodedcrystal plate may be mounted and electrically connected by cementing orotherwise firmly attaching fine conductive supporting wires directly toa thickened part of the electrodes of the crystal element at its nodalpoints only. Such fine supporting wires may be secured to the electrodedcrystal element by conductive cement and may extend horizontally fromthe vertical major surfaces of the crystal element and at their otherends be attached by solder, for example, to vertical conductive wires orrods carried by the press-or other part of an evacuated or sealed glassor metal tube. The supporting wires and rods may have one or more bendstherein to resiliently absorb mechanical vibrations. Also, bumpers orstops of soft resilient material such as mica may be spaced closelyadjacent the edges, ends or other parts of the electroded crystalelement in order to limit the bodily displacement thereof when thedevice is subjected to mechanical shock. In a suitable mounting of thistype for the crystal-element, the horizontal supporting wires may bespaced along the vertical rods to suit the nodal points of theelectroded crystal elements, It will be understood that any holder whichwill give stability, substantial freedom, from spurious frequencies anda relatively high Q or reactance resistance ratio for the crystalelement may be utilized for mounting the crystal element.

two of the three mutually perpendicular X, Y and Z axes thereof, themajor axis length dimension of said major surfaces being in said planeand intermediate said two of said three X, Y and Z axes, the dimensionalratio of the width dimen sion of said major surfaces with respect tosaid length dimension thereof being one of the values betweensubstantially 0.2 and 1.0, and means including a plurality of sets offunctionally independent electrodes adjacent said major surfaces foroperating said element simultaneously at a plurality of independentlycontrolled frequencies dependent upon different sets of saidmajorsurface dimensions, one of said frequencies being dependent upon thefundamental of the longitudinal or extensional mode vibration along saidwidth or length dimension.

4. A piezoelectric Rochelle salt type crystal element having itssubstantially rectangular major surfaces substantially parallel to theplane of two of the three mutually perpendicular X, Y and Z axesthereof, the major axis length dimension of said major surfaces being insaid plane and intermediate said two of said three X, Y and Z axes, thedimensional ratio of the width dimension of said major surfaces withrespect to said length dimension thereof being one of the valuessubstantially in the region of 0.5, and means including a plurality ofsets of functionally inde- I Although this invention had been describedand illustrated in relation to specific arrange ments, it is to beunderstood that it is capable of application in other organizations andis therefore not to be limited to the particular embodiments disclosed,but only by the scope of the appended claims and the state of the priorart.

What is claimed is:

1. A face mode piezoelectric Rochelle salt type crystal element havingits substantially rectangucrystal element having its substantiallyrectangular major surfaces substantially parallel to the planeof an Xaxis and the Z axis, the major axis length dimension of said majorsurfaces being inclined atan angle of substantially 42 26' with respectto said X axis, the dimensional ratio of the width dimension of saidmajor surfaces with respect to said length dimension thereof being oneof the values between substantially 0.2 and 1.0.

3. A'piezoelectric Rochelle salt type crystal ele-' ment having itssubstantially rectangular major surfaces substantially parallel to theplane of pendent electrodes adjacent said major surfaces for operatingsaid element simultaneously at a plurality of independently controlledfrequencies dependent upon said major surface dimensions, one of saidfrequencies being dependent upon the fundamental of the longitudinal orextensional mode vibration along said width dimension, and another ofsaid frequencies being dependent upon the second harmonic of thelongitudinal or extensional mode vibration along said length dimension.

5. A piezoelectric Rochelle. salt type crystal element adapted tovibrate simultaneously at 'a plurality of desired independentlycontrolled 'longitudinal mode frequencies one of which is dependentmainly upon the length dimension and another of which is dependent uponthe width dimension of its substantially rectangular major surfaces,said length dimension being substantially in the plane of a Y axis andthe Z axis and inclined at an angle of substantially 49 56' with respectto said Y'axis, said major surfaces being substantially parallel withrespect to said YZ plane, the ratio of said width dimension of saidmajor surfaces with respect to said length dimension thereof being oneof the values within the region of substantially 0.50.

' 6. A piezoelectric Rochelle salt type crystal element adapted tovibrate simultaneously at a plurality of desired independentlycontrolled longitudinal mode frequencies one of which is dependentmainly upon the length dimension and another of which is dependent uponthe width dimension of its substantially rectangular major surfaces,said length dimension being substantially in the plane of an X axis andthe Z axis and inclined at an angle of substantially 42 26' with respectto said X axis, said major surfaces being substantially parallel withrespect to said xz plane, the ratio of the width dimension of said majorsurfaces with respect to said length dimension thereof being one of thevalues within the region of substantially 0.50, said width dimensionexpressed in centimeters being a value of substantially 118 divided bysaid another of said frequencies expressed in kilocycles per second.

7. A piezoelectric Rochelle salt type crystal 'elementadapted to vibratesimultaneously at aplurality of independently controlled frequenciesdependent mainly upon different sets of the length and width dimensionsof its substantially rectangular major surfaces, said length dimensionbeing substantially in the plane of aYaxis and the Z axis and inclinedat an angle of substantially 49 56' with respect to said Y axis, saidmajor surfaces being substantially parallel with respect to said YZplane, the ratio of the width dimension of said major surfaces withrespect to said length dimension thereof being one of the values Withinthe range from substantially 0.2 to 1.0, said width dimension and saidlength dimension being a set of corresponding values in accordance withthe values of sai frequencies.

8. A piezoelectric Rochelle salt type crystal element adapted to vibratesimultaneously at a plurality of desired independently controlledfrequencies dependent mainly upon different sets of the length and widthdimensions of its substantially rectangular major surfaces, said lengthdimension being substantially in the plane of an X axis andthe Z axisand inclined at an angle of substantially 42 26' with respect to said Xaxis, said major surfaces being substantially parallel with respect tosaid XZ plane, the

ratio of the Width dimension of said major surfaces with respect to saidlength dimension thereof being one of the values within the rang fromsubstantially 0.2 to 1.0, said width dimension and said length dimensionbeing a set of corresponding values in accordance with the values ofsaid frequencies.

9. A piezoelectric Rochelle salt type crystal element and meansincluding two functionally independent pairs of opposite electrodescooperating with said element for vibrating said elementsimultaneously-at two independently controlled desired frequenciesdependent mainly upon different sets of the length and width dimensionsof the rectangular major surfaces of said element, said length dimensionbeing substantially in the plane of a Y axis and th Z axis and disposedat an angle of substantially 49 56' with respect to said Y axis, saidmajor surfaces being substantially parallel with respect to said YZplane.

10. A piezoelectric Rochelle salt type crystal element and meansincluding two functionally stantially parallel to the plane of a Y am'sand a Z axis thereof, and said length dimension being inclined at anangle of substantially 49 56' with respect to said Y axis to obtain saiddesired longitudinal mode frequency substantially free from couplingwith other modes of motion in said crystal element.

12. A Rochelle salt crystal element in accordance with claim 11 whereinsaid desired longitudinal mode frequency'is a fundamental modefrequency, and said length dimension expressed in centimeters is equalto substantially 160 divided by said desired frequency expressed inkilocycles per second.

13. A Rochelle salt crystal element in accordance with claim 11 whereinsaid desired longitudinal mode frequency is a harmonic mode frequency,and said length dimension expressed in centimeters is equal tosubstantially 169 divided by said desired frequency expressed inhiccycles per second, and multiplied by the numerical order of saidharmonic frequency.

14. A Rochelle salt type piezoelectric crystal element adapted to vibratat a desired longitudinal or extensional mode frequency, said crystalelement having substantially rectangular shaped majorsurfaces, saidmajor surfaces having a length or longer dimension and width or shorterdimension, said width dimension being made of a value in accordance withthe value of said desired frequency, said major surfaces and said lengthand width dimensions being substantially parallel to th plane Of a Yaxis and a Z axis thereof, and said length dimension being inclined atan angle of substantially 49 56' with respect to said Y axis to obtainsaid desired longitudinal mode frequency substantially free fromcoupling with other modes of -motion in said crystal element.

15. A Rochelle salt crystal element in accordance with claim 14 whereinsaid desired longitudinal mode frequency is a fundamental modefrequency, and said width dimension expressed in centimeters is equal tosubstantially 160 divided by said desired frequency expressed inkilocycles per. second.

16. A Rochelle salt type piezoelectric crystal element adapted tovibrate at a desired longitudinal or extensional mode frequency, saidcrystal element having substantially rectangular I shaped majorsurfaces, said major surfaces havindependent pairs of oppositeelectrodes cooperating with said element for vibrating said elementsimultaneously at two independently controlled desired frequenciesdependent mainly upon different sets of the length and width dimensionsof the'rectangular major surfaces of said element, said length dimensionbeing subing a length or longer dimension and width or shorterdimension, said length dimension being made of a value in accordancewith the value of said desired frequency, said major surfaces and saidlength and width dimensions being substantially parallel to the plane ofan X axis and a Z axis thereof, and said length dimension lieinginclined at an angle of substantially 42 26' stantially in the plane ofan X axis and the Z axis and disposed at an angle 01 substantially 4226' with respect to said X axis, said major surfaces being substantiallyparallel with respect to said XZ plane.

11. A Rochelle salt type piezoelectric crystal element adapted tovibrate at a desired longitudinal or extensional mode frequency, saidcrystal element having substantially rectangular shaped major surfaces,said major surfaces having a length or longer dimension and width orshorter dimension, said length dimension being made of a valu inaccordance with the value of said desired frequency, said'major surfacesand said length and width dimensions being subwith respect to said Xaxis to obtain said desired longitudinal mode frequency substantiallyfree from coupling with other modes of motion in said crystal element. 7

17. A Rochelle salt crystal element in accordance with claim 16 whereinsaid desired longitudinal mode frequency is a fundamental modefrequency, and said length dimension expressed in centimeters is equalto substantially 118 divided by said desired frequency expressed inkilocycles per second.

' .18. A Rochelle salt crystal element in accordance with claim 16wherein said desired longitudinal mode frequency is a harmonic modefrequency, and said length dimension expressed in centimeters is equalto 118 divided by said desired frequency expressed in kilocycles persecond, and multiplied by the numerical order of said harmonicfrequency.

19. A Rochelle salt type piezoelectric crystal element adapted tovibrate at a desired longitudinal or extensional mode frequency, saidcrystal element having substantially rectangular shaped major surfaces,said major surfaces having a length or longer dimension and width orshorter dimension, said width dimension being made of a value inaccordance with the value of said desired frequency, said major surfacesand said length and width dimensions being substantially parallel to theplane of an X axis and a Z axis thereof, and said length dimension beinginclined at an angle of substantially 42 26' with respect to said X axisto obtain said desired longitudinal mode frequency substantially freefrom coupling with other modes of motion in said crystal element.

20. A Rochelle salt crystal element in accordance with claim 19 whereinsaid desired longi tudinal mode frequency is a fundamental modefrequency, and said widthdimension expressed in centimeters is equal tosubstantially 118 divided by said desired frequency expressed inkilocycles per second.

WARREN P. MASON.

